VOLCANIC ACTIVITY AND ERUPTIONS

Volcanic activity ranges from emission of gases, non-explosive lava emissions to
extremely violent explosive bursts that may last many hours. The types of
eruptions determine the relative volumes and types of volcaniclastic
material and lava flows, consequently the shapes and sizes of volcanoes.

A volcanic event occurs when there is a sudden or continuing
release of energy caused by near-surface or surface magma movement.
The energy can be in the form of earthquakes, gas-emission at the
surface, release of heat (geothermal activity), explosive release of
gases (including steam with the interaction of magma and surface of
ground water), and the non-explosive extrusion or intrusion of magma.
An event could be non-destructive without release of solids or
magmatic liquid, or if there is anything to destroy, could be destructive with voluminous lava flows or
explosive activity. Destruction usually refers to the works of mankind (buildings, roads, agricultural land, etc.).

A volcanic event can include

(1) an eruptive pulse (essentially an explosion with an
eruption plume, but also non-explosive surges of lava. A pulse may last a
few seconds to minutes,

(2) an eruptive phase that may last a few hours to days and
consist of numerous eruptive pulses that may alternate between
explosions and lava surges, and

(3) a single eruption or eruptive episode, composed of
several phases, that may last a few days, months or years
(Fisher and Schmincke, 1984).
Paricutin, Mexico was in eruption for nine years. Stromboli, Italy
has been in eruption for over 2000 years.

Simkin et al. (1981) define eruptions in terms of inactive
periods. An eruption that follows its predecessor by less than 3
months is considered to be a phase of the earlier eruption unless it is
distinctly different (explosive versus effusive, different magma
type).

Some volcanoes (e.g., domes and basaltic scoria cones) may form
completely within a few weeks or months. Others, such as shield
volcanoes and composite volcanoes may show high order discontinuities
such as major chemical changes, volcano-tectonic events like caldera collapse, or long erosional intervals, and may last 10 m.y. or more
before volcanism completely dies out.

During a single eruption, styles of activity and types of products
may change within minutes or hours, depending upon changes in magma
composition, volatiles, or other magma chamber and vent conditions.

Types of Eruptions

Volcanic eruptions and eruptive phases are traditionally
classified according to a wide range of qualitative criteria; many have
been given names from volcanoes where a certain type of behavior was
first observed or most commonly occurs.

Common eruptions types are Plinian, Hawaiian, Strombolian, and
Vulcanian. Gas-only eruptions are not so common.

Gas emissions or "eruptions"

On August 21, 1986 in the Cameroon highlands, West
Africa, Lake Nyos emitted carbon dioxide that moved like a river down-valley
for 110 kilometers and suffocated 1200 people in the town of Nyos, and, in
nearby villages of Subum and Cha, more than 500 died. In addition
3000 cattle died along with all preditors and insects.

At the present time, carbon dioxide gas is seeping upward through
Mammoth Mountain, a composite volcano on the edge of Long Valley Caldera.
The carbon dioxide leaks
are occurring at several places around the volcano. Long Valley and
Mammoth Mountain are being watched by the U.S. Geological Survey.
It is not known whether or not the CO2 leaks could be a precursor
to a volcanic eruption.

Plinian Eruptions

Widely dispersed sheets of pumice and ash are derived from high
eruption columns that result from high-velocity voluminous gas-rich
eruptions, commonly lasting for several hours to about four days. These are
called Plinian from
Pliny the Younger who described the famous 3-day
eruption of Vesuvius in 79 AD during which the towns of Pompei and
Herculaneum were buried by several meters of pyroclastic material from
Vesuvius.
Plinian eruptions commonly produce high eruption columns. The energy
and characteristics of a Plinian eruption depends on gas content of the
magma, exit pressure, viscosity, vent radius and shape, and volume of magma
erupted. Most Plinian eruptions result from explosions of highly evolved
rhyolitic to dacitic, trachytic and phonolitic magmas with temperatures from
about 750 to 1000 Celsius.

(This article is still "in progress")

<1>ERUPTION COLUMNS1>

Sparks et al. (1978) concluded that a pyroclastic flow develops
around the base of a collapsing eruption column, deflates, and then
moves outward across the landscape under its own momentum. In their
model, the momentum that a pyroclastic flow acquires is proportional
to the height from which the eruption column collapses.

The conclusion that momentum is the main cause of transport of
pyroclastic flows influenced the "energy line" concept of Sheridan
(1979). Sheridan explained that the slope of the energy line as proposed by
Hsu (1975) for avalanche runout, traces the potential flow head from
the top of the gas-thrust region of an eruption column to the distal
toe of a flow along the line of transport. However, only a tiny
fraction of the total fragmental component reaches the top of the
gas-thrust part of an eruption column. Most of the fragmental
material that falls back is located between the top of the gas-thrust
and the ground surface. The total momentum acquired cannot be a
single mass number that attains a particular height, but a summation
of all the fragmental mass and the different heights to which they
attain. Thus, the calculated momentum value is exaggerated.

McEwen and Malin (1989) argued that the energy-line model
predicts velocities that are too high, resulting in flow paths that
are insufficiently responsive to topography. They suggest that
velocity-dependent resistance factors such as Bingham or turbulent
models are needed for accurate velocity predictions.
Viscosity-dependent factors, however, are less effective resistance
factors than internal processes that effect sediment gravity flow such
as flow transformations, density stratification, decoupling and
blocking, particularly in mountainous regions. These processes need
to be considered as resistance factors that affect the forward
progress of flows. Fisher (1990), for example, shows that mountainous
terrain itself can be considered as a roughness element that
significantly effects runout distance.

Pyroclastic flows can originate in several ways. It is generally
accepted that the main origin is by gravitational collapse of a
vertical eruption column (Sparks and Wilson, 1976; Sparks et al.,
1978). The collapse of vertical eruption columns as a process in the
origin of pyroclastic flows was first recognized by Kozu (1934) and
discussed by Smith (1960a) and Williams (1942). Collapse of columns
was postulated from sedimentological data by Hay (1959a).

"Bulk subsidence" (column collapse) was suggested as the cause for
development of pyroclastic flows by Fisher (1966b). The term bulk
subsidence was used for the processes that led to the base surge
development from an underwater atom bomb explosion at Bikini Atoll
(Brinkley et al., 1950). Bulk subsidence of an eruption column was
described from a series of photographs showing the development of a
base surge at Capelinhos (Azores) (Waters and Fisher, 1971).

The connection between column collapse and the origin of
pyroclastic flow and surge deposits was firmly established by Sparks
and Wilson (1976) and Sparks et al. (1978), but it must be pointed out
that bulk subsidence and column collapse are visualized as two
different processes.
Development of nues ardentes by column collapse was recognized
during the 1969 eruption of Mayon volcano (Phillipines) (Moore and
Melson, 1969), during the 1974 eruption of Ngauruhoe Volcano (New
Zealand) (Nairn et al., 1976; Nairn and Self, 1978). Devastation on
all sides of El Chichon volcano (Mexico) occurred during one of the
eruptive episodes of its 1982 eruption (Sigurdsson et al., 1984,
1986), and on all sides of Mt. Pele (Martinique) during its 30 August
30 1902 eruption.
Observations of base surge and nuee ardente development, coupled
with well reasoned theoretical arguments on the criteria for column
collapse (Wilson, 1976; Sparks and Wilson, 1976; Sparks et al., 1978;
Wilson et al., 1980; Wilson and Walker, 1987) and numerical modeling
(Valentine and Wohletz, 1989) has established the validity of the
process.
Some pyroclastic flows and surges, however, have originated
without development of high vertical eruption columns, such as the
1951 eruption of Mount Lamington (Papua) (Taylor, 1958). Several times
during its eruption, convoluted clouds filled the crater but had
little tendency to rise; instead, the heavier parts flowed through low
gaps in the crater wall whereas the lighter parts poured over the
crater rim. Wolf (1878) reported an eruption at Cotopaxi (Ecuador)
which "glowing lava" "boiled over" from the crater and flowed with
furious velocity in all directions down the slopes, a description
similar to that given by Taylor (1958) to some of the flows at Mount
Lamington. Activity similar to the "massive disgorgements" at Mount
Lamington also occurred during at least one eruptive episode on July
9, 1902 at Mt. Pele (Anderson and Flett, 1903, p. 492-493).
During the 18 May eruption of Mount St. Helens, following the the
blast phase, most of the pumiceous pyroclastic flows formed when
"bulbous masses" of inflated ash, lapilli and blocks erupted to a few
hundred meters like a fountain above the inner crater and then spilled
out through the open crater to the north (Rowley et al., 1981). These
upwellings took place before or during the development of the gas
thrust of the Plinian column that occurred without visible column
collapse. Pyroclastic flows and surge development also preceded the
vertical eruption column during the 22 July and 7 August 1980
eruptions of Mount St. Helens (Hoblitt, 1986). Each eruptive pulse
began with a fountain of gases and pyroclasts around the vent that
generated a pyroclastic density current. The change from fountaining
to vertical column activity is interpreted to be caused by an increase
in the gas content of the eruption jet or else a decrease in vent
radius with time.
Sparks et al. (1978) postulated that pyroclastic flows originate
following the fall-back of a turbulent, collapsing eruption column and
then move outward as a non-turbulent flow. Their calculations, using
flow velocities ranging from 10 to 200 m/s, a drag coefficient of
0.01, and terminal velocity measurements of pyroclastic particles by
Walker (1971), showed that grains >1 mm could not be carried in
suspension.
The runout length of pyroclastic flows and their ability to
surmount topographic barriers are topics of continuing research
germane to the distribution of ignimbrite sheets. Pumice-rich
pyroclastic flows are known to have crossed topographic barriers of
considerable height (Yokoyama, 1974; Miller and Smith, 1977; Koch and
McLean, 1975; Rose et al., 1979). The 22,000 yr B.P. Ito pyroclastic
flow (Japan) traveled 70 km over topographic barriers as high as 600 m
(Aramaki and Ui, 1966; Yokoyama, 1974). The 18,000 B.P. Taupo
Ignimbrite, only ~30 km3 in volume, is spread out over a ~20,000 km2
area and mantles mountains as high as 1500 m above the inferred vent
as far as 45 km from the source (Wilson, 1985; Wilson and Walker,
1985). Pyroclastic flows from Aniakchak and Fisher calderas in the
Aleutian Islands traveled as far as 50 km over mountainous barriers
between 250 and 500 m high (Miller and Smith, 1977).
Currently, there are two general models that describe the way
that pyroclastic currents move across the landscape -- (1) as expanded
flows (EFs) thicker than the height of the mountains they traverse, or
(2) as dense flows (DFs) moving as a nonturbulent ground-hugging sheet
across the landscape (Sparks, 1976). The purpose of the present paper
is to test these ideas by analyzing the stratigraphy and flow
directions, as determined by anisotropy of magnetic susceptibility
(AMS) measurements (see below), of the Campanian Tuff.
The two models require fundamentally different hydrodynamic
behaviors. For EFs, the pyroclastic current must remain turbulent to
maintain its expansion to a thickness greater than the topography that
it overtops. Also, an expanded current can travel across water
because expansion reduces its bulk density so that only its basal part
interacts with the water (Sigurdsson et al., 1991). Dense pyroclastic
flows probably cannot easily travel above water. Moreover, should
they enter and travel beneath water, viscous boundary effects, mixing
with water, and other conditions would inhibit flow, and it is
unlikely that they could re-emerge.
One critical observation applicable to the problem discussed
herein is that pyroclastic currents such as nues ardentes are known to
separate gravitationally into a lower part containing most of the
solid fragmental mass. This natural density stratification in
initially turbulent pyroclastic currents (Valentine, 1987) and other
sediment gravity flows (Fisher, 1983, 1984) commonly results in flow
transformations from turbulent to nonturbulent behavior in basal
zones where concentration values become high. Having different
densities and turbulent behaviors, the different parts of the current
can decouple and travel different paths, depositing material
independently (Fisher, 1990). In mountainous terrain, flow
transformations, decoupling and divergent flow directions are
especially amplified. Study of these effects can contribute to a
better understanding of pyroclastic flow emplacement processes.
Valentine (1987) concluded that pyroclastic flows may become
density stratified and do not necessarily completely collapse to a
non-turbulent condition of flow. According to his model, density
stratification does not necessarily form a surface above which is
mostly gas and below which is a dense flow, but rather there is a
continuous gradation from one to the other. At flow velocities of
r100 m/s and r300 m/s, particles as large as 1 cm and 10 cm
repectively can be turbulently supported -- considerably larger than
sizes calculated by Sparks et al. (1978). The differences in
supportable clast sizes stem from the choices of substrate roughness
and boundary layer thickness. Valentine (1987) proposed a rougher
terrain than Sparks et al. (1978), with roughness elements (such as
tree stumps) up to 1 m. Sparks et al. (1978) assumed a flat terrain
with a roughness of 1 cm and considered the whole flow as a boundary
layer.
The EF model contends that a pyroclastic current may initially be
of medium- to low-density, but unlike the DF model, it remains
expanded as it travels over the landscape leaving behind a
depositional carpet deposited from its basal part, a model proposed by
Fisher (1966) and extended by Branney and Kokelaar (1992). Flow of
DFs is based upon a plug flow model whereby deposition is thought to
occur by en masse freezing of the debris, similar to nonvolcanic
debris flows, rather than layer by layer accretion (Sparks, 1976).
Pyroclastic eruptions commonly produce eruption columns that transport
volcaniclastic particles from beneath the ground into the atmosphere.
The eruption column is a gas-solid dispersion that is columnar-shaped
and extends into the atmosphere from the surface vent. The physical
properties and dynamic processes within eruption columns affect many
physical attributes of pyroclastic deposits. Moreover, the different
properties of eruption columns define the diverse styles of
classically defined pyroclastic eruptions.
Sustained explosive volcanic eruptions into the atmosphere
commonly produce volcanic plumes. Many features of their origins,
shapes, and dynamic behavior have become quantitatively known only in
the past decade (Wilson, 1976; Blackburn et al., 1976; Sparks and
Wilson, 1976; Settle, 1978; Wilson et al., 1978; Sparks, 1986;
Valentine and Wohletz, 1989). Dispersal of fragments from them are
becoming clarified (Carey and Sparks, 1986; Wilson and Walker, 1987).
The height of eruptions columns (up to 50 km) and wind vectors
determine particle distributions from volcanic eruptions (Chap. 6)
Eruption columns are divided into three main components: the lower or
gas thrust part, a central convective thrust part (Wilson, 1976;
Blackburn et al., 1976; Sparks and Wilson, 1976; Wilson et al., 1978),
and an upper part known as the umbrella region (Sparks et al., 1986;
Sparks, 1986).
Expansion of juvenile volcanic gas, and, in Vulcanian eruptions,
pressure from expanding steam are the driving forces for the gas
thrust part. Collapse criteria have been based upon the effects of
exit velocity, gas content, vent radius (Sparks et al., 1978; Wilson
et al., 1980; Wilson and Walker, 1987), but an important parameter is
also shown to be the effect of exit pressure (Valentine and Wohletz,
1989) based upon numerical modelling. The numerical modelling by
Valentine and Wohletz (1989) also suggests that column behavior is
much more sensitive to the exit pressure ratio than to the density
ratio between the column and the atmosphere.
Widely dispersed sheets of pumice and ash are derived from high
eruption columns that result from high-velocity voluminous gas-rich
eruptions, commonly lasting for several hours to about four days (Fig.
4-4). These are called Plinian because Pliny the Younger described
the famous 3-day eruptions of Vesuvius in 79 AD during which the towns
of Pompei and Herculaneum were buried by several meters of fallout
pumice, followed by pyroclastic surges and flows (Sigurdsson et al.,
1985) that are an integral part of many Plinian deposits (Figs. 8-21,
8-31). Plinian fallout is commonly associated with voluminous
pyroclastic flow deposits from calderas (Chaps. 6, 8). Here we
briefly summarize some features of the controls of eruptive processes
in Plinian and related eruptions (Walker et al., 1971; Walker, 1973;
Wilson, 1976, 1980; Sparks and Wilson, 1976; Sparks et al., 1978,
Wilson et al., 1978, 1980).
The energy and characteristics of a Plinian eruption depends on
many factors, among which gas content of the magma, exit pressure,
viscosity, vent radius and shape, and volume of magma erupted are
especially important. Most Plinian eruptions result from explosions
of highly evolved rhyolitic to dacitic, trachytic and phonolitic
magmas with liquidus temperatures from about 750x to 1000xC. Thus, a
mean temperature of 850xC is assumed in the following discussion
(Wilson et al., 1980). The eruption velocity, Uv, is nearly
proportional to the square root of temperature. This enables
adjustments for different temperatures. Density is assumed to be
2.3x103 kg m-3, and volatile content about 5 weight percent,
dominantly water, as discussed in the previous section. The viscosity
is about 104 to 107 Pas (rhyolite).
Wilson et al. (1978) have shown that maximum column height, H, is
proportional to the fourth root of the mass eruption rate. If 70
percent of the heat released by the erupted material is used to drive
convection, then
H = 236.6 m1/4.
Wilson et al. (1980) have discussed three combinations of vent
radius and gas content in monitoring exit velocities in column height
(Fig. 4-7). They show that while velocity drastically decreases with
decreasing gas content, column height is mainly dependent on vent
radius. Column collapse, at conditions of constant vent radius equal
to 200 m, only occurs when water contents drop below 2.4 percent.
Widening vent radius may also lead to reverse grading commonly
reported in Plinian deposits. An initial Plinian phase will be
followed by pyroclastic flows when either gas content decreases or the
vent widens (Fig. 4-8). It should be noted, however, that numerical
modeling by Valentine and Wohletz (1989) suggests that the formation
of a Plinian column does not require entrainment and heating of
atmospheric air, and the pressure effects that they present do not
support the assumption that column behavior is determined entirely by
the efficiency of air entrainment. Salient features of Plinian type
eruptions and their products are summarized in Table 4-2.
Carey and Sigurdsson (1986) have developed a model of pyroclastic
dispersal that discriminates between eruption column height and
transport by local winds based upon the geometry of particle isopleth
maps constructed from field measurements. They used the model to
calculate eruption column height and from that, eruption intensity
based on the behavior of convective plumes under a variety of
atmospheric conditions. Eruption intensity is defined as the
volume-rate at which magma is discharged. They (Carey and Sigurdsson,
1989) further show that peak eruption intensities (i.e., magma
discharge rate) is positively correlated with the magnitude (total
erupted mass; all erupted products). Initial Plinian fall phases with
intensities > 2.0 x 108 typically precede the onset of a major
pyroclastic flow (chap. 8) and caldera subsidence. During eruptions
of large magnitude, the transition to pyroclastic flows is likely to
be the result of high intensity, whereas the generation of pyroclastic
flows in small magnitude eruption s may occur more often by reduction
of magmatic volatile content or other transient changes in magma
properties. As shown in figure 4-8, transitions from a convecting
column to a collapsing column can occur by two different trajectories
shown by arrows: (1) large increases in intensity or vent size or (2)
decrease in volatile content or exit velocity of magma. Carey and
Sigurdsson (1989) suggest that caldera-forming events which generate
large-volume pyroclastic flows follow path (1) and small-volume
pyroclastic flow may occur along path (2).

Hawaiian and Strombolian Eruptions

Hawaiian eruptions consist of basaltic, highly fluid lavas of
low gas content, that produce effusive lava flows and some pyroclastic
debris. Thin, fluid lava flows can gradually build up large broad shield
volcanoes. Most Hawaiian eruptions start from fissures, commonly beginning
as a line of lava fountains that eventually concentrate at one or more
central vents. Most of the vesiculating lava falls back in a still molten
condition, coalesces and moves away as lava flows. If fountains are weak,
most lava will quietly well out of the ground and move away from a vent as a
lava flow. Much lava in shield volcanoes is transmitted through tubes
enclosed within lava flows. Small spatter cones and, in some instances,
basaltic pumice cones such as at Kilauea Iki, may form around vents.
Pyroclastic material occurs as bombs, ranging downward in size through
lapilli-sized clasts of solidified liquid spatter commonly called cinders,
to small volumes of glassy Pele's tears and Pele's hair.

Strombolian eruptions, named after Stromboli Volcano, Italy, are
discrete explosions separated by periods of less than a second to several
hours. They give rise to ash columns and abundant ballistic debris. Ejecta
consist of bombs, scoriaceous lapilli and ash.
Stromboli, and other Italian volcanoes are described in Boris Behncke's page on
Stromboli.

Klyuchevskaya volcano, Kamchatka in eruption. Typical Strombolian event.
From post card of the National Geographic Society.

Vulcanian Eruptions (hydrovolcanic)

Vulcanian eruptions are from
hydrovolcanic processes (Fisher and Schmincke, 1984). Many volcanologists
use the term Vulcanian for highly explosive, short-lived eruptions that produce
black, ash- and steam-laden eruption columns as witnessed during the 1888-90 eruptions
of Vulcano, a small volcano in the Eolian Islands, Italy (see e.g. MacDonald,
1972).

The Complex Multiple Eruptive Behavior of Mount St. Helens

From the World Wide Web page site of the U.S. Geological Survey, David
Johnston Cascades Volcano Observatory.

The 1980 eruptive episode of
Mount St. Helens included more than one
type of eruptive behavior and more than one kind of volcanic hazard. It is
not uncommon for volcanoes to exhibit a range of eruptive types during an
eruption.